Selecting balls in tennis before serving?

Question

I watched a lot of tennis matches and I never realized why the tennisers choose the ball they want to serve with. Are the balls different? I don't think so, so why?

Answer 1

You often see tennis players inspecting the balls before choosing a ball to play with because they are checking for the newest ball, the ball with less wear. New balls get the best bounce.

Answer 2

Two main reasons go into chosing a ball the way you see them do.

First- They will probably be looking for the newest ball (without scuffs from the court), because when the ball is in play the air pressure inside is being knocked out.  Tennis balls lose their pressure the more they are played, as you can probably guess that the newest ball has the biggest bouce.  

Second- Tennis being an obsessive-compulsive sport, the pro may be wanting to use the same ball he just won the recent point with, a "lucky" ball.  This is why you may see Federer or other pros ask the ball-retriever for the one they just used.